1. Technical Field of the Invention
This invention relates to a marine propulsion and steering unit.
2. Prior Art
More particularly, the invention relates to a marine propulsion and steering unit of the kind comprising a pod having front and rear ends, a driving machine accommodated in the pod, a substantially horizontal propeller shaft drivingly connected to the driving machine and provided with a propeller externally of the front end of the pod, and an upright pod supporting strut rigidly attached to the pod and having at the upper end thereof swivel bearing means supporting the pod supporting strut and the pod below a buoyant body for angular motion about a substantially vertical axis.
The buoyant body may be a ship, a work platform, a pontoon, or a similar floating body.
A propulsion and steering unit of this kind is known from EP-B-0 394 320. Because the unit is angularly movable about a vertical axis, it may be used not only for the propulsion, but also for the steering of the ship or other buoyant body equipped with the unit, and at the same time the rudder can also be used for the steering. Angular adjustment of the entire unit may also be combined with deflection of the rudder.
If the buoyant body equipped with the unit is a ship adapted to be run at a high speed, 20 knots or more, for example, the unit will be subjected to very great forces by the water if it is turned while the ship is running at such high speed. The swivel bearing and the actuators and other components used for the turning of the unit will therefore be heavily stressed during steering manoeuvres. When the ship is running at a high speed, steering by means of the rudder is therefore preferred. Steering by turning the entire unit, possibly combined with deflection of the rudder, is resorted to when running at a lower speed, such as when the ship is manoeuvred in harbours or narrow waterways.
In the known unit, the pivotal axis of the rudder coincides with the vertical turning axis of the unit. In certain operating conditions, this arrangement of the turning axes causes hydrodynamical problems which are related to the position of the rudder and the inhomogeneous flow of the water impinging on the unit when the unit is angularly offset from the fore and aft vertical centre-line plane of the ship.